1. Field of the Invention
The invention relates to a method for forming a thin film transistor (TFT) of an organic light-emitting display (OLED), and more particularly, to a method for forming a TFT that can solve the problem of threshold voltage shifting of the gate of a TFT.
2. Description of the Prior Art
Since the progress of science and technology has led to small, effective, and portable intelligent information products, display devices have played an important role in modern society. In recent years, display devices have undergone great improvements in the areas of high performance quality, larger size, and lower cost. Among the various display devices in the display market, OLEDsare poised replace liquid crystal displays (LCD) and cathode ray tube (CRT) displays, as they have advantages of simpler structures, self-emitting design, excellent operating temperature, high definition, high contrast, and a wide viewing angle. OLEDs can be applied to mobile phones, personal digital assistants (PDAs), hand-held videogames, digital cameras, portable DVD players, and automobile navigation devices.
An OLED uses organic luminous devices, such as organic light-emitting diodes, as the light source of the display. An organic luminous device is an electrically driven lighting element having a brightness that depends on the magnitude of a related current. At present, the magnitude of the brightness (which is also called the gray-scale value) is controlled by the magnitude of the driving current in an application organic light-emitting matrix display. As a result, the OLED utilizes this characteristic of the organic luminous material to generate red, blue, and green lights with different intensities of gray level to produce stunning images.
Based on the driving method, the matrix display can be classified as either a passive matrix or an active matrix display. Passive matrix displays adopt the method of driving the scan lines of the display in sequence, driving pixels in different rows sequentially. Since the light-emitting time of each pixel is restricted by the scanning frequency and the numbers of scan lines, the passive matrix method is not suitable for large-sized and high dots-per-inch (dpi) displays with a high amount of scan lines. Active matrix displays, however, possess an independent pixel circuit for each pixel, which includes a capacitor (Cs), an organic light-emitting component, and at least two TFTs that are used to adjust the OLED driving current. With this arrangement, even in large-sized and high dpi displays, a steady driving current is provided for each pixel, which improves the brightness balance.
FIG. 1 is a schematic diagram of an active matrix OLED panel 10 according to the prior art. As shown in FIG. 1, a display panel 12 comprises a matrix composed of a plurality of data lines 22 (such as DL1, DL2, and DL3) and scan lines 24 (such as SL1 and SL2). The display panel 12 also comprises a plurality of pixel circuits 26 having TFTs, a storage capacitor (Cs), and organic luminous devices (not shown) at each intersection of a data line 22 and a scan line 24. Each pixel circuit 26 electrically connects to a corresponding data line 22 and a corresponding scan line 24 for driving the organic light-emitting diode 20 in the corresponding pixel. The data lines DL1, DL2, and DL3 connect to an external data line driver 16 for receiving an image data signal, and the scan lines SL1 and SL2 connect to an external scan line driver 18 for receiving a switch/address signal.
FIG. 2 is a schematic diagram of the pixel circuit 26 shown in FIG. 1. As shown in FIG. 2, the pixel circuit 26 comprises a first TFT 28, a second TFT 30, and a storage capacitor 32. In the prior art, generally, the first TFT 28 and second TFT 30 are NMOS transistors. The gate of the first TFT 28 is electrically connected to the scan line 24, and the source, point A, of the first TFT 28 is electrically connected to the data line 22. In addition, the gate, point B, of the second TFT 30 is electrically connected to the source of the first TFT 28 and one end of the storage capacitor 32. And the source, point C, and the drain, point D, of the second TFT 30 are electrically connected to an external power supply Vdd and the organic luminous device 20 respectively.
The driving method of the conventional OLED 10 is described in the following. Referring to FIG. 1 and FIG. 2, when a video data signal is inputted into a control circuit 14, the control circuit 14 generates corresponding control signals to the data line control circuit 16 and the scan line driving circuit 18 according to the video data of each pixel. Then, the scan line driving circuit 18 outputs corresponding scan signals to each scan line 24 (SL1, SL2, . . . and SLn) in sequence for turning on the pixel circuits 26 in each row in order and thereby making the corresponding pixels perform the display operation. For example, when the OLED 10 is going to drive a pixel positioned in the intersection of DL3 and SL3, the control circuit 14 sends a corresponding data signal, normally a voltage signal with a predetermined intensity, to the drain of the first TFT 28 in the pixel circuit 26 through the data line driving circuit 16 and the data line 22 according to the video data.
Since the first TFT 28 and the second TFT 30 both conduct, the current from the data line 22 will charge the storage capacitor 32 with the first TFT 28. After that, the storage capacitor 32 has a first voltage and generates a corresponding driving current at point C, which is then output to the organic luminous device 20 to make the organic luminous device 20 generate light beams with a corresponding brightness. When the OLED 10 performs in continuous operation, such as driving the pixels in the next row, the storage capacitor 32 still has the first voltage although the voltage on SL3 decreases resultingin the first TFT 28 becoming closed. Therefore, the second TFT 30 still conducts. Furthermore, since there is a voltage difference between point D and point C, a current is continuously passing through the second TFT 30 to the organic luminous device 20 to continuously keep the organic luminous device 20 emitting light beams. Since a pixel circuit may have various design structures, the amount of TFTs in a pixel circuit may be different. However, a pixel circuit contains at least two TFTs for driving the organic luminous device.
In the driving method of an OLED as described above, the pixel circuit used for driving the organic luminous device is one of the key devices for displaying video data on time and correctly. In addition, since the TFT is the main element for conducting the pixel circuit and controlling the organic luminous device to continually emit light, normally the quality of a TFT is a key factor in the performance of an organic light emitting display.
Generally, the TFTs of the pixels in the prior-art OLEDS are NMOS transistors. According to the prior art, the process of fabricating such an NMOS transistor is to form a gate on a glass substrate, to deposit a gate insulating (GI) layer, an amorphous silicon layer, and a dopedn+ layer in sequence, and to remove portions of the doped n+ layer and the amorphous silicon layer to define the gate pattern. Then, an indium tin oxide (ITO) layer is formed on the glass substrate. After that, a portion of the ITO layer is removed to form the pixel electrodes. Following that, a metal layer is formed on the doped n+ layer, and a photoetching-process (PEP) is performed to remove a portion of the metal layer and the doped n+ layer to form a source and a drain, and to simultaneously expose a portion of the amorphous silicon layer. Finally, a passivation layer is formed on the source and the drain, and then a portion of the passivation layer is removed to finish the fabrication of the TFT of an OLED.
However, according to the prior art, the amorphous silicon layer is composed of silicon molecules that are not crystallized, so that the amorphous silicon layer has low electron mobility. Furthermore, the threshold voltage of the gate of a TFT with amorphous silicon material will shift depending on the fabrication process, which will result in each TFT having an uncertain gate threshold voltage. In this situation, various driving currents will be output resulting in uneven brightness, as each organic luminous device will have a differing gray-scale value. This is especially so when the TFT is under a long-time operation, as electrons may remain in the amorphous silicon layer. As a result, a voltage stress on the TFT is generated, leading to an increased uncertainty in the threshold voltages of gates that seriously degrades the display quality of the OLED.
To avoid such threshold voltage shifting, a great majority of OLED manufactures use low temperature polysiliconthin film transistors (LTPS TFT). However, the process for fabricating an LTPS TFT is more complicated and has significantly higher costs. As a result, how to produce a TFT with a stable gate threshold voltage using simple processes and using a low-cost hydrogenated amorphous silicon layer (α-Si:H layer) is an important issue.